1. Field of the Invention
The invention relates to a ceramic heater suitable in an application such as a system for producing semiconductors.
2. Related Art Statement
In a system for producing semiconductors, a ceramic heater may be provided for heating a wafer so as to deposit a semiconductor thin film on the wafer from gaseous raw materials such as silane gas by means of thermal CVD, plasma CVD or the like. In such a ceramic heater, it is required to make the temperatures on the mounting face and the semiconductor wafer mounted thereon uniform with high precision.
Several techniques for reducing the temperature distribution on the heating (mounting) face of such a ceramic heater are known. For example, a so-called two-zone heater is known. Such a two-zone heater includes a ceramic substrate and inner and outer resistance heat resistors made of a high melting point metal that are embedded in the substrate. Separate power supply terminals are connected to the respective heat resistors so that electric power may be applied independently on the respective resistors. Heat generated from the inner and outer heat resistors may be thus independently controlled.
Such a two-zone heater includes the following. Japanese patent publication 2001-102157A discloses a heater having a ceramic substrate and two layers of heating elements embedded in the substrate. The calorific values in the inner and outer zones of each heating element are controlled so that a two-zone control system of controlling inner and outer zones may be realized.